A booster is known in the art which comprises a tubular valve body slidably disposed within a shell, an annular, first valve seat formed around the inner peripheral surface of the valve body, a valve plunger slidably fitted inside the valve body, an annular, second valve seat formed on the valve plunger, a valve element which is urged by a spring disposed within the valve body so as to be seated upon either the first or the second valve seat, a constant pressure passage providing a communication between a portion which is located radially outward of a first seat defined by the contact between the first valve seat and the valve element and a constant pressure chamber which is partitioned within the shell, a pressure passage for providing a communication between a portion which is located radially inward of a second seat defined by the contact between the second valve seat and the valve element and a source of pressure fluid, and a variable pressure passage for providing a communication between a portion located intermediate the first and the second seat and a variable pressure chamber which is also partitioned within the shell.
In the inoperative condition of the booster described above, the valve plunger is retracted to cause the valve element to be seated upon the second valve seat and to be spaced from the first valve seat. Under this condition, the second seat defined by the contact between the second valve seat and the valve element is closed to thereby close the pressure passage which communicates with the internal portion thereof while the first seat defined by the contact between the first valve seat and the valve element is opened to allow a communication between the constant pressure passage, located outside the first seat, and the variable pressure passage, located inside the first seat. In this manner, the constant pressure chamber communicating with the constant pressure passage and the variable pressure chamber communicating with the variable pressure passage assume an equal pressure, whereby the booster is inoperative.
By contrast, when the booster is operated, the depression of a pedal causes the valve plunger to be advanced, whereupon the valve element is seated upon the first valve seat to close the first seat, thus interrupting a communication between the constant and the variable pressure chamber. A continued advancement of the valve plunger causes the valve element to be removed from the second valve seat to open the second seat, whereupon the pressure passage, located inside the second seat, communicates with the variable pressure passage, located outside the second seat, allowing pressure fluid to be introduced into the variable pressure chamber to create a pressure differential between the constant and the variable pressure chamber to operate the booster.
When the booster is operated, that is, when pressure fluid is introduced into the variable pressure chamber through the pressure passage, sounds may be acoustically generated. To accommodate for this, it has been a practice to provide an acoustic absorbing material adjacent the inlet of the pressure passage to present a resistance to the passage of pressure fluid therethrough in an attempt to reduce the rate at which the pressure fluid is introduced into the variable pressure chamber in order to suppress the generation of sounds. However, such muffler suffers from a disadvantage that the response of the booster is degraded inasmuch as the provision of the acoustic absorbing material within the pressure passage presents a resistance to the passage of pressure fluid therethrough.